A plate heat exchanger comprises a plate pack consisting of a number of assembled heat transfer plates forming between them plate interspaces. Generally, every second plate interspace communicates with a first inlet channel and a first outlet channel, each plate interspace being adapted to define a flow area and to conduct a flow of a first fluid between said inlet and outlet channels. Correspondingly, the other plate interspaces communicate with a second inlet channel and a second outlet channel for a flow of a second fluid. Thus, the plates are in contact with one fluid through one of their side surfaces and with the other fluid through the other side surface, which allows a considerable heat exchange between the two fluids.
Modern plate heat exchangers have heat transfer plates, which in most cases are made of sheet-metal blanks that have been pressed and punched to obtain their final shape. Each heat transfer plate is usually provided with four or more “ports” consisting of through holes punched in the four corners of the plate. Sometimes, additional ports are punched along the short sides of the plates so as to be located between the ports punched in the corners. The ports of the different plates define said inlet and outlet channels, which extend through the plate heat exchanger transversely of the plane of the plates. Gaskets or any other form of sealing means are arranged round some of the ports alternatingly in every second plate interspace and, in the other plate inter-spaces, round the other ports so as to form the two separate channels for the first fluid and the second fluid, respectively.
Since considerable fluid pressure levels are obtained in the heat exchanger during operation, the plates need to be sufficiently rigid so as not to be deformed by the fluid pressure. The use of plates made of sheet-metal blanks is possible only if the plates are somehow supported. Generally, this is achieved by the heat transfer plates being provided with some kind of corrugation so that the plates bear against each other in a large number of points.
The plates are clamped together between two flexurally rigid end plates (or frame plates) in a “frame” and thus form rigid units with flow channels in each plate interspace. The end plates are clamped together by means of a number of clamp bolts, which engage both plates by the intermediary of recesses or holes made along the circumference of each end plate.
In recent years, plate heat exchangers have come into use in applications in which at least one of the fluids is subjected to a phase change (condensation or evaporation). In many processes, vapour is used for heating purposes for two reasons: on the one hand vapour contains a lot of energy that is released upon condensation and, on the other hand, the heating temperature is essentially constant. In the case of condensation temperatures exceeding 100° C. the temperature cannot be regulated by means of, for instance, a so-called vapour trap, which regulates the pressure of the condensate discharged. In the case of temperatures below 100° C. vapour traps do not work for natural reasons—no pressure can be achieved below atmospheric pressure. Instead, a condenser in which residual vapour is condensed must be used.
Conventional plate heat exchangers are not very well adapted for this task because of their symmetric design; ports of the same size and the same channel characteristic in both channels. In a typical application, the relationship between the vapour flow and the flow of cooling water is such that the diameter of the vapour inlet should be twice as large as that of the cooling water ports. Moreover, the channels in the plate inter-spaces should be highly asymmetric. The vapour requires a channel with a large cross-sectional area and low frictional resistance in order for the pressure drop to be minimized, and the cooling water requires a narrow channel with large frictional resistance that causes heavy turbulence.
In applications of this kind, the plates should have relatively large vapour ports to prevent the vapour phase pressure drop at the port or ports from becoming too great, which would have a detrimental effect on the efficiency of the heat exchanger. To allow for the provision of ports in a plate heat exchanger of the type mentioned above the plates have to be wide. This implies poor utilisation of the sheet-metal, which in turn makes the plate heat exchanger too expensive.
In this context, also the type of plate heat exchanger as described in DE-A1-19716200 should be mentioned. This publication discloses a plate heat exchanger in which all ports, i.e. also the ports for the different fluids, are positioned along one and the same line. The object stated in the DE publication is that it is desirable to obtain an improved distribution of the flow over the width of the heat transfer plates. The shape of the plate is essentially long and narrow and rectangular, and the two ports' for one of the fluids are positioned at the outer end of each short side of the plate whereas the two ports for the other fluid are positioned inside the same.
Furthermore, GB 2121525 discloses an evaporator or condenser made up of plates having respectively a long and narrow upper port and a long and narrow lower port intended for a first fluid, which is to be conducted to every second plate interspace. The two ports extend over the whole width of the plate. The plate further comprises a number of protrusions which are disposed outside the width of the plate and which each consist of a thin sheet-metal ring enclosing the corresponding port. These ports are intended to conduct a second fluid to the other plate interspaces. However, in this construction the frame plates must be of considerable size, since they need to extend over the whole width of the plate as well as the protrusions.
Nor does the design disclosed in U.S. Pat. No. 4,523,638 satisfy the design requirements stated above regarding efficient utilisation of the sheet-metal. Moreover, in this US publication the ports are disposed in conventional manner, i.e. one port in each corner. The preamble to claim 1 is based on such a conventional condenser.
Finally, EP 411,123 discloses a special type of falling-film condenser in which the inlet port and the outlet ports for the liquid are disposed adjacent to the lower edge. This particular type of condenser is intended for processes involving heat sensitive products, such as fruit juice, unrefined sugar solutions or the like, and does not provide any solution to the problems related above.